474 



LIEUT.-GENEBAL SABINE ON TERRESTEIAL MAGNETISM. 



Table XVII. 



Corrections for the infiuence of the ship's iron on the observations of the Intensity of the 



Magnetic Force. 



The corrections to be applied to the intensity observations of the third year of the 

 Survey have been derived from observations which were made with great care at Simon's 

 Bay (Cape of Good Hope) in the ' Terror' on the 7th of April 1843, and in the ' Erebus* 

 on the 12th of the same month, with the ship's head placed successively on each of 

 the sixteen principal points as indicated by the standard compass. The arithmetical 

 mean of the sixteen determinations has been regarded as a result in which the disturbing 

 influences on the several points may be considered to have balanced each other, and as 

 a true measure of the Force at the locality. The correctness of this conclusion was 

 further established by the removal of the instruments from the ships to the shore, where 

 the results obtained accorded well with the mean of the observations on the sixteen 

 points on board. 



In the formation of tables for the correction of the individual observations made at 

 sea in the course of the year's survey, the values of the coefficients a, b, c, and d, deter- 

 mined by the investigations in the preceding parts of this communication, have been 

 employed, viz. 



In the 'Erebus' . . fl= - -0331 ; *=+-9875 ; c=-f 0-0195 ;<?= +0-9936. 

 In the 'Terror'. . . o=~-0344: 5=+-9901,- c=+0-0106; rf= + 0-9950. 



With these values tables of double entry were formed, having as arguments 6, and 

 tabular values of ^; employing for the Dip Corrections equations (12) and (13) 

 (Philosophical Transactions, 1843, Art. X. p. 148) ; and for the Intensity Corrections 



A'c( - tan ^-f- cos M cos ^ cosec^ (Philosophical Transactions, 1843, Art. X. p. 162); 



A' in the ' Erebus' being found =1-0051, and in the ' Terror' =1-0055. 



The application of these corrections to the observations with the ship's head on the 

 different points at Simon's Bay, in April 1843, is shown in the following Table. 



